Electric circuits



Sept. 9 1924.

F. E. FIELD ELECTRIC CIRCUITS Filed Nov. 1, 1921 Cyc/es per Second Patented Sept. 9, 1924.

UNITED STATES PATENT OFFICE.

MK EDWIN FIELD, OF SOMERVILLE, NEW JERSEY, ASSIGNOR TO WESTERN ELEC- TRIO COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

Application filed November To all whom it may concern:

Be it known that I, FRANK E. FIELD, a citizen of the United States, residing at Somerville, in the county of' Somerset, State of New Jersey, have invented certain new and useful Improvements in Electric Circuits, of which the followin is a full, clear, concise, and exact description.

This invention relates to electric circuits and more particularly itrelates to means for reducing the distortion in a repeating system.

One object of this invention is to reduce the distorting effect due to undesired and inherent capacity reactances in translating devices such as transformers and vacuum tube repeaters.

Another object is to secure a desired shape of the curve which corresponds to the relation between the amplification or gain of a vacuum tube repeater and the frequency of the currents impressed thereon. This last object also comprehends reducing the distortion produced by a"repeater circuit, due either to the inherent capacity between the electrodes of the repeater or to the capacity between and in the windings of repeating coils, or both.

In the case where the vacuum tube repeater is coupled to the incoming line by means of a repeating coil or transformer, these objects are accomplished in accordance with this invention by employing a non-uniform core member for the transformer whereby the transformer is given an effective inductance due to the leakage flux of such a value that the inductance in connection with the capacity above mentioned resonates at a desired frequency. It has been found that for voice current transmission, the preferred value for this frequency is near the upper limit of the important voice frequencies and may have a value of approximately 5000 cycles per second. In this case, it has been found that the curve which corresponds to the gain of the repeater for different frequencies of transmitted current is substantially flat throughout the important part of the voice frequencyrange, which means that the ain is constant over that range and the deslred reduction in distortion is secured.

As set forth in United States application of W. L.-Casper, Serial No. 462,883, filed April 20, 1921, for Electric circuits, po-

ELECTBIC CIRCUITS.

1, 1921. Serial 1T0. 512,147.

tential variations impressed on the control electrode of a vacuum tube, when power is supplied to the input circuit, can be influenced by the electrostatic capacities between the electrodes of the tube, and in ordinary tubes the effective input reactance, as measured between the filament and the control electrode, or grid, is the reactance of a capacity greater than the electrostatic ca acity between the filament and the grid by an amount proportional to the electrostatic capacity between the grid and plate, increasing with the amplificationconstant of the tube and decreasing with the resistance in the output load. This effective input reactance is inversely proportional to the frequency. Therefore, when the tube is fed from a circuit containing impedance which does not decrease with the frequency, the amplification obtained, as measured by the ratio of the voltage output to the voltage input, will ordinarily decrease as the frequency is increased. The general effect of the effective input capacity of the tube is consequently to decrease the effective input impedance of the tube and therefore to decrease the amplification of the repeater when the frequency becomes high, thereby causing distortion due to unequal amplification at difierent frequencies. In the ordinary tubes, the distorting effect of this internal input electrode capacity increases with the frequency and becomes appreciable in the neighborhood of 1000 cycles. It has also been found in the case where the vacuum tube repeater is connected to the incoming line by a transformer that further distortion or a lowerin of the gain of the repeater occurs due to e ectrostatic capacity effects between and in the windings of the transformer. It appears that these undesirable effects of the transformer capacity are caused by the reactance 'of this capacity acting as an effective shunt across the grid and the filament, this capacity being, in effect, in parallel with the internal capacity of the tube. The effective internal capacity of tubes in ordinary use varies from 20 to 250 mmf. depending on the type of tube and the output impedance into which it is working. Input transformers in ordinary use have capacities varying between 15 and 350 mmf.

This invention will be better understood by reference to the following detaileddescription taken in connection with the acformer ha'vin companying drawings in which 1 represents this mvention embodied in a trans-- a two-mesh core; Fig. 2

As is well known in the art, the standard practice of manufacturing such core struc tures as 10 has heretofore been to make the cross section of arm 13 equal to the sum of the cross sections of arms 14 and 15. This, of course, gives a uniform magnetic field for the flux and insures a negligible amount of leakage.

In accordance with this invention, how

'6 V61, the standard practice has not been followed, but one of the portions of the core structure has been given a cross section less than that necessary to produce a uniform magnetic field, thereby producing an appreciable amount of leakage. In Fig. 1, for example, the arm 13 is shown to be of considerably less cross section than the sum of the crosssections of the two members 14 and 15, it bein assumed that the third dimension of t ese three members (not shown) is the same. ,As described above, the

, resultant leakage flux acts as an inductance in circuit with the transformer which resonates with the transformer capacity and the internal capacity of the tube of the repeater circuit to bring about the desired modification of the repeater gain characteristic.

The transformer core 10 may be made of magnetic'material such as iron formed in any of the ways known in the art. It may consist, for example, of superposed iron laminations, some of which may not be entirely circumferentially complete, in order to provide the usual small air gaps in the core structure and also to provide for a convenient assembl In onecase w ere it was desirable to have a flat gain characteristic over a frequency 'range from 1000 to 3000 cycles approximately, it was found necessary to make the cross section of arm 13 two-thirds the value of the cross section of arms 14 and 15, in

the case where the effective transformer ca-' pacity was approximately 25 mmf. and the effective internal capacity of the tube was ap roximately 30 mmf.

s described in the Casper application above referred to, it may be desirable at frequency approximately 5000 cycles.

with either an upward or a'downward slope with rise in frequency instead of oneyconstant with fre uency. In such cases, the rise in gain wit fre uency can be obtained by increasing the lea age inductance, while a decrease in gain with increase in frequency can be obtained-by decreasing the leakage inductance. Fig. 3 illustrates various types of curves which may be obtained'for repeater amplifications over a range of frequencies up to 5000 cycles. The curve, 20 illustrates the gain characteristic obtained by the ordinary repeater in the absence of the leakage inductance correcting means of this invention, while curve 21 represents the type of gain obtainable by employing arm 13 with the cross section two-thirds the value of the sum of the cross sections of times to have. a repeater gain characteristic proximately two-thirds the cross section of portion 24 in order to obtain thefiat gain characteristic under the conditions noted above.

Fig. 4 represents one way in which the transformer of this invention may be associated with the vacuum tube repeater, the gain of which is to be modified. The input transformer 25 which connects the vacuum tube repeater 26 with an incoming line 27 i may be provided with a core member of the type shown in Fig. 1 in order to give the repeater a. flat characteristic'curve over the frequency range of importance-present in the, incoming signals or other. currents in line 27. If line 27 is a telephone line on which speech frequency currents occur, it follows that the repeater 26 will amplify these currents in accordance with curve 21 of Fig. 3. The am lified currents may, of course, be impresseii going line 28 by an output transformer 29.

In general, it may be said that the leakupon a suitable outage inductance in the transformer should be of such a value. as to resonate with the transformer capacity and the tube capacity for a frequency of the order of but eater than the highestlfrequency for 'whlch the repeater gain is to be regulated. Thus in case the frequency range of im ortance is from 10,00 to 3000 cycles, it wil generally be found desirable to have the leakage inductances and the capacities resonate atIfa however, it is desired to transmit one frequency or a narrow band of frequencies, the undesired effect of the grid-cathode capacity and transformer-capacity may be neutralized by a leakage inductance of such a value as to resonate therewith for such a frequency that the amplification-frequency curve of the repeater has a maximum point at the frequency to be transmitted.

It is to be understood that the arrangement for amplification control set forth herein with .especial reference to a voice frequency range may also be applied to amplification control at other frequencies as, for example, frequencies commonly used for carrier currents.

What is claimed is:

1. In combination, a translating device having a capacity reactance, and means associated therewith for modifyin the effect of said reactance on currents for a wide range of frequencies transmitted through said device to provide a substantially constant attenuation for said range comprising a transformer with a core having an arm, the cross section of which is appreciably smaller than it should be as compared with other portions of said core for a uniform flux in said core.

2. In combination, an amplifying device having an impedance comprising ,a resistance component and a'reactance component, a line, a transformer for connecting said line to said device, said transformer comprising a core of such a non-uniform character as to cause a substantial leakage of flux therein.

3. In combination, an amplifying device having a resistance component and a reactance component, a line, a transformer for connecting said line to said device and comprising a core having a reduced cross section in one part thereof for causing such a flux leakage as-to reduce the distortion of currents received form said line.

4. In combination, two line sections and means for coupling said lines to provide a substantially constant transmission between said sections for a wide range of frequencies, saidmeans comprising -a transformer between said line and having a core of such a reduced cross section at one portion thereof as to cause an appreciable leakage flux.

5.'In combination, a, line, an amplifying device, and means for giving said device an approximately flat gain characteristic over a wide range of frequencies present in said line, said means comprising a transformer associated with said device and having a core of such a reduced cross section at one portion thereof as to cause an appreciable leakage flux.

6. In combination, a vacuum tube repeater having an impedance comprising a resistance component and'a capacity component, and means for substantially annulling the effect of said capacity component on frequencies transmitted through said re eater, said means comprising a transformer aving a substantially circumferentially complete core member of such a configuration as to cause sufiicient flux leakage to give rise to an effective inductance resonating with said capacity.

In combination a line, a translating device, and a trans ormer associated with said line and said device and having an appreciable capacit between its windings, said transformer havmg a substantially circumferentially complete core member of such a configuration as to produce a suificient leakage ux to annul the effect of said capacity on currents of a wide range of frequencies transmitted throu h said transformer.

8. In combination, a telephone line, a vacuum tube repeater having an impedance comprising a resistance component and a capacity component, and means for giving said repeater an approximately flat gain characteristic from 1000 cycles to 3000 cycles, said means comprising a transformer interconnecting s a1d line and said repeater and having a core member within the windings of said transformer, a core member external to said windings and joined to the ends of said first core member, one of said core members havin a reduced cross section over what it should ave for a uniform flux in said members, said reduction in the cross section being sufiicient to give an effective inductance resonating with the capacity of said repeater and the capacity between the windings of said trans ormer for a frequency in the neighborhood of 5000 cycles.

In witness whereof, I hereunto subscribe my name this 27th day of October A. D., 1921.

FRANK EDWIN FIELD. 

